Bottom! :D
OK, you need to understand some anatomy. There is no reverse kegel muscle. And the bulbocavernosus (better called the bulbospongiosus) or BC doesn’t stop urine flow. That’s just a way to get guys to understand what it feels like.
The urinary bladder has a sphincter muscle (a circle) that surrounds the urethra as it leaves the bladder. In its resting state it is closed and compressing the urethra so urine cannot flow. To urinate you consciously (most of the time, if you’re passed out drunk and someone puts your hand in a bucket of water, all bets are off), contract the sphincter - so it opens - and urine flows. To make urine flow faster you compress your abdominal muscles and put pressure on the pelvic contents which includes the bladder. This forces urine out of the bladder faster.
The BC has nothing to do with urine except that little squirt at the end when you finish. It’s pushing the last bit of urine from the urethra just as it pushes semen out during ejaculation. But consciously stopping urine flow involves the same nerves that are used to release the urinary sphincter and allow it to close around the urethra. That’s why “stopping urine flow” is what guys new to kegels are instructed to do so they know what it feels like.
As you may know the pubococcygeus (PC) is more involved with the expulsion of feces. It’s major function is to pull the sigmoid colon back into the body after one pushes against it with increased intra-abdominal pressure while having a bowel movement. However, the nerves involved in that are connected to the ones used for stopping urination and usually they all contract at the same time.
If you’re up to what I think you are then increasing intra-abdominal pressure may be what you should try to master rather than major anal muscle contractions. It will simply take a lot of practice to get the fine control that you’re looking for. It is possible to develop a sense of control over the individual nerves, but it won’t be something learned in a day or two.
